In sand core making machines, the cores are made from a mixture containing sand and a binder. The mixture is introduced in a core box defining a cavity (or cavities) with the shape of the core (or cores) to be made. The core box is usually made up of two facing forming implements, defining the cavity therebetween. A single core can be made in one and the same core box (a cavity is defined), or a plurality of cores can be made simultaneously (a plurality of cavities is defined).
Each cavity defined in a core box is filled with the mixture used for making the core. Before introducing said mixture in the corresponding cavity, said cavity is full of air that must be discharged to allow said mixture to be housed in said cavity. To that end, the core box comprises at least one outlet conduit communicating the cavity with the outside of the core box, through which said air is discharged to the outside (to the environment) as said mixture is gradually being introduced. A filter is furthermore arranged in the outlet conduit to prevent said mixture from being discharged therethrough. Said outlet conduit is usually a through hole of the lower implement.
Once the required mixture is in the corresponding cavity, said mixture is hardened to provide rigidity to the core such that it can then be used where and as required.
The binder used may be organic or inorganic. For hardening mixtures with organic binders, a catalyst such as amine, for example, is usually applied on the mixture, and in some cases pressurized hot air can be used (usually together with the catalyst). Due to the properties of the binder or the catalyst, contaminant gases are generated during mixture hardening and a subsequent treatment or process on said gases is required. Furthermore, due mainly to the cost and danger of catalysts of this type, said catalysts are supplied in a controlled and metered manner, requiring a more complex and costly control over the catalysts compared to the control required over air (when air is used).
For hardening mixtures with inorganic binders, it is usually sufficient to apply pressurized hot air on the mixture drying said mixture by absorbing at least part of its moisture, hardening it, without contaminant gases being generated during the process (moist air is usually generated as a result of moisture absorption).
In hardening processes using pressurized hot air, the air used is dry and hot so that it absorbs moisture from the mixture present in the cavity, such that said mixture is hardened. In addition to this, it is common to heat the core box so that the actual heat of said core box also absorbs part of the moisture of the mixture. The air is generally supplied from a pressurized air source and conducted to the core box, passing it beforehand through a heating device for heating it, such that it reaches said core box (and therefore the corresponding cavity) hot. Since it is hot, it is able to absorb moisture from the mixture present in the core box, and the higher the temperature the air has as it enters the core box, the higher the absorption capacity it will have. However, the more the values of these properties of the air increase, the higher the cost that will be entailed in the core making process (particularly due to the energy requirements necessary for achieving high temperatures).
In processes of this type, there is furthermore a need to assure a minimum air pressure at the air inlet in the core box to assure that said air reaches the entire mixture present in the cavity. If the air reaches it at a low pressure, there is a risk of it not reaching the center of said mixture, for example, with the risk this entails in making fragile cores (the center is not hardened in this case), and/or of it not reaching all the cores suitably (if a plurality of cores is made simultaneously in one and the same core box). Therefore, pressure regulators are usually arranged between the pressurized air source and the heating device to assure that the air is supplied at least with the required minimum pressure. When replacing one core box with another, air pressure can be regulated to a new desired value, if required, given that each of the core boxes can have different needs, and this regulated pressure is maintained as long as the core box is not changed, not being modified during a core making cycle.
The pressurized hot air which is introduced in the core box for hardening the mixture present therein must be discharged as it is introduced so that the moisture of the mixture is discharged from the core box and said mixture is properly hardened. The outlet conduit (generally a plurality of outlet conduits) of the core box through which air present in the corresponding cavity is discharged as the mixture is introduced therein is normally utilized for this discharge, said pressurized and already moist hot air thereby being discharged from the core box through said outlet conduit.
Finally, the core thus made is removed from the core box, and the core box is ready to start another making cycle.
Patent document EP1849537A1 discloses a sand core making machine, comprising a core box in which an inorganic mixture which is subsequently hardened with pressurized hot air is introduced. The machine comprises a heating device between the source and the core box, and a proportional pressure valve arranged between said source and said heating device for regulating air pressure. To improve hardening process efficiency, this machine comprises two alternative air paths from the source to the core box which are selected in a controlled manner depending on the moment of said process. The air is first passed through a heating unit of the heating device with a specific heating capacity, and the air is then passed through said heating unit and two other additional heating units of said heating device arranged in series with the first, thereby significantly increasing air temperature and therefore air moisture absorption capacity. This, however, requires a high overall energy consumption, despite the fact that energy only needs to be provided to one heating unit in the first phase, and furthermore requires a complex installation given that there is a need to provide two different air paths, which makes maintenance difficult and increases the cost thereof.
US20160250680A1 discloses a method for producing cores or molded parts, in which a basic molding material is mixed with alkali silicate or water binder and a lost core or a molded part is formed using a core shooter in a core box.
US20030173049A1 discloses a sand core making machine comprising a core box, a blowing device suitable for introducing a sand-containing mixture in the core box, and a hardening device suitable for introducing pressurized hot-air in the core box, conducted through a specific path, for hardening the mixture present in said core box.